The guys hate the kid due that the kid is rubbing against Nami's breasts, being a pervert, and Nami is not even aware the kid is doing this pervert rubbing against her. The guys are jealous that this kid can get away with such a pervert thing.

People hating on the kid are missing the point. Momonosuke is committing his experiences to memory for future reference. He knows what's up. Why else would the losers in the second panel be weeping in envy?